Identifying Crystal Structure Characteristics to Facilitate the Development of Rare-Earth Substituted Inorganic Phosphors
Solid state lights (SSLs) have great potential to reduce energy consumption while limiting environmental impact since they have high efficiencies, long lifetimes, and contain environmentally benign materials. The prototypical SSL produces white light by combining a blue light emitting diode (LED) with an inorganic phosphor, a host structure substituted with a luminescent center such as Ce3+ or Eu2+. The LED emission is partially absorbed by the phosphor and down-converted to longer wavelengths; the combination of the LED and phosphor emission yields a broad spectrum, white light. Although functional, the lack of a red emission component results in a cold, blue-tinted white light that is unacceptable for most in-home and color sensitive applications. Alternatively, a warmer white light can be formed by combining a UV LED with three inorganic phosphors (red, green, and blue) that completely down-convert the LED emission providing better coverage of the visible wavelength region. However, because of the large Stokes’ shift between the phosphor excitation and emission, the phosphors must be incredibly efficient, as measured by the internal photoluminescent quantum yield (Φ). Due to a shortage of such phosphors, research is now focused on identifying crystal structure characteristics that influence emission full width at half the maximum (FWHM), Φ, and thermal stability to facilitate targeted synthesis of potential candidates. Previous research has shown that crystal structures with highly symmetric and ordered local environments often have restricted emission FWHM and high Φ. The best descriptor to identify ordered, rigid structures is the Debye temperature (ΘD), which is typically determined using ab initio calculations on the unsubstituted host structure with the knowledge that including the luminescent center may affect this value. A high ΘD indicates a compound is likely to possess high Φ. However, many compounds with high ΘD have low Φ, indicating that ΘD does not account for thermal quenching. These results highlight the need for further efforts to account for thermal behavior through computational techniques, already proven vital in correctly solving crystal structures. Combining knowledge of the structural features that dictate luminescence behavior with computational tools will allow for targeted synthesis of new rare-earth substituted inorganic phosphors.